Papovaviruses are small DNA tumor viruses that induce a number of human diseases, including cancer. Our long-term goal is to analyze SV40 virion protein function in molecular terms, from mechanisms of virus assembly to spread of DNA tumor viruses. SV40 has a unique morphogenetic pathway that begins with targeting of an infectious viral particle's genome to the nucleus, where it directs the synthesis of early gene products followed by late gene expression. The mRNAs for the structural proteins are transported to the cytoplasm and translated there. Vps form cytoplasmic protein complexes (cytoplasmic subvinion assembly) and are transported back into the nucleus, where the viral genome is now packaged together with host histones for virion assembly (nuclear vinion assembly). Although extensive information exists on this morphogenesis, important details of subvirion and virion assembly and transport of the protein complexes to the nucleus remain unclear. Since papovaviruses, particularly papillomaviruses, use similar pathways, studies in the SV40 model system will have general implications critical to the understanding of carcinogenetic events in humans. Together with cellular proteins and virally encoded early-gene products, specific structural motifs of SV40 viral structural proteins (VP1, VP2/3) are beleived to direct each step of the subvinon and vinion assembly processes and virus dissemination. We obtained a unique set of mutants whose defects prevent the progression to the next step of SV40 morphogenesis in proper 1) Vp1 folding, 2) Vp1-Vp2/3 interaction, 3) virion assembly, and 4) infection of the next host.Using our knowledge of these mutations, we propose to elucidate the molecular basis of viral and cellular functions critical for SV4O morphogenesis. The specific aims for the next 5 years are to: 1) delineate how VP1 assembles in the cytoplasm by examining VP1 mutants defective in VP1 folding and pentamer formation; 2) describe how heterotypic VP 1 -VP2/3 complexes form in the cytoplasm and determine requirements for the assembly of these proteins; 3) define mechanisms of minichromosome packaging and processes of infectious particle formation in the nucleus; 4) analyze signals for the nuclear entry of virions in achieving the next infection cycle. The proposed studies will provide us with a solid molecular foundation for a deeper understanding of morphogenesis of DNA tumor viruses in general and of SV40 in particular.